Standards such as IEEE 802.11 (ANSI/IEEE Std 802.11, 1999 edition) specify the Medium Access Control (MAC) and PHYsical (PHY) layer characteristics for wireless local area networks (WLANs).
PHY specifications define a plurality of PHY modes for use in the transmission of data frames. Each PHY mode uses a particular modulation and channel coding scheme and, consequently, offers different performance in terms of transmission duration, overhead, and robustness against reception noise and interference.
In wireless methods, such as WLANs, the propagation environment changes over time and space due to factors such as mobility and interference. For that reason, no PHY mode can be regarded as optimal under all possible conditions.
In WLANs, frame-reception errors are caused by the combination of noise and interference at the receiver side. Generally, the average packet loss rate, Ploss, is determined by the modulation and coding scheme (PHY mode) adopted and by the signal-to-noise-and-interference ratio (SNIR) at the receiver side. In the absence of interfering signals, the frame error rate depends on the propagation-channel characteristics with respect to the PHY mode adopted.
In that case, the average packet error rate is denoted by Perr, and Ploss can be expressed as follows:Ploss=Perr+Pcoll−Pcoll*Perr  (1)where Pcoll accounts for the frame error probability due to collisions, i.e., signals overlapping at the receiver side. The third term in the right-hand side of (1) accounts for the fact that the two error events (channel error and collision error) are not disjoint, since packet errors can be determined by the combination of the effects of noise and interference.
Link adaptation (LA) procedures have been proposed to adjust the PHY_mode and, in turn, Ploss, to maximize the data throughput (goodput), S, given by the product of the MAC-layer data rate, MAC_Data_rate, and the frame success probability:S=MAC_Data_rate*(1−Ploss)  (2)where both terms depend on the adopted PHY_mode. In fact, MAC_Data_rate generally increases (sublinearly) with the PHY rate, while Ploss generally increases at the increasing of the PHY rate, given that the SNIR remains unchanged.
Certain prior art LA procedures attempt to set the PHY mode according to the estimated channel quality, which is measured mainly through fluctuations in the Received Signal Strength Indicator (RSSI).
However, it has been noted that RSSI provided by commercial devices is often imprecise and/or inaccurate and may impair the performance of such procedures. Furthermore, in case of asymmetric radio propagation conditions, the RSSI measured by the transmitting station might not reflect the Signal to Noise Ratio (SNR) experienced by the receiving station.
Other prior art LA procedures, such as Auto Rate Fallback (ARF), exploit the fact that a failed transmission attempt can be recognized by the non-reception of an acknowledgement (ACK) frame, which is normally sent by the receiver after correct reception.
These procedures assume that all transmission failures are ascribable to adverse propagation conditions, i.e., they assume that Ploss˜=Perr. Therefore, a certain number of subsequent failures is attributed to a lowering of the SNR and a more robust rate is selected. Conversely, when a certain number of subsequent successful transmissions are observed, a higher rate is selected to improve throughput.
It has been noted that a drawback of this approach lies in that, under contention-based MAC schemes and medium-to-high traffic loads, transmission failures might also be caused by MAC collisions, irrespective of the actual propagation conditions, that is to say Ploss>Perr. As a consequence, under heavy traffic conditions, these procedures are likely to unnecessarily decrease the rate even if the signal strength would be able to support a higher and better-performing modulation scheme, thus further exacerbating the medium contention problem. Indeed, rate reduction implies larger frames, with the effect that medium congestion worsens. It has thus been noted that, in that sense, ARF mechanisms are suboptimal since they do not take into account the causes of frame losses and can lead to wrong decisions when the number of contending users is appreciable, such as for example in hotspots, airports, and SOHO environments.
Further, it has been noted that another drawback of ARF-like selection procedures is given by unnecessary oscillations of the rate value, which may vary in response to the activity of other user's participating in the network. Since ARF-like procedures can not distinguish losses due to collisions and losses due to poor link quality, they are subject to modifying the rate even if the Signal-to-Noise ratio remains constant: if for example a burst transmitter is contending with a given wireless LAN client, the latter is likely to drop the rate when the first is transmitting and, vice versa, increase the rate when it is not. This effect is highly undesirable since it has harmful impact on the upper layer protocols (TCP/IP, UDP/IP) which see a time varying available bandwidth.
This effect may be especially problematic for high-bandwidth demanding multimedia applications.
For instance, FIG. 1 is an exemplary diagram representative of collision, error, and success events in a time vs. rate space for a node in a WLAN moving form 0 m at 50 s to 100 m at 100 s, by assuming 10 interferers with an ideal signal-to-noise ratio (SNR).
FIG. 1 refers to an ideal rate selection procedure in the following scenario: a mobile station is exchanging data with an 802.11g access point and moves away from it over time at constant speed, thus decreasing its SNR. The access point also serves an other ten fixed contending stations placed close to it. The Y axis shows the ideal rate selection procedure over time. In these circumstances, the rate remains fixed for given time spans and varies in response to changes in the SNR (i.e., the distance of the mobile station from the access point).
FIG. 2 is a thoroughly similar diagram representative of operation of an ARF procedure. In this case, the selected rate spans several values for the same time slots: the choice of the ARF procedure is not stable but oscillates with a rather high swing for a given signal to noise ratio.
Also, it has been noted that current Wireless LAN cards use a rate selection mechanism driven by MAC statistics, which, however, are unable to differentiate between a noisy link and a congested network. As a consequence, in the presence of congestion, these conventional WLAN card use a low PHY rate for transmission, which results in longer frames, with the result of making network congestion even worse.
Finally, it has been noted that current rate selection procedures are “blind” insofar as they adapt the modulation scheme simply based on the number of frame retransmissions, no matter what the cause of frame loss is.